Method of manufacturing silver platelets

ABSTRACT

The present invention provides an aqueous solution-based method for producing nano-sized silver platelets, which employs the controlled mixing of a silver ion solution, a reducing solution, and an acidic solution, under suitable conditions. Also provided are the silver platelets produced thereby, and compositions containing the silver platelets.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional of and claims priority to Applicant's co-pending U.S. patent application Ser. No. 11/913,284, filed Oct. 31, 2007, which is a national stage filing under 35 U.S.C. 371 of PCT Application Ser. No. PCT/US2006/061492, filed on Dec. 2, 2006, which claims the benefit of U.S. Provisional Application Ser. No. 60/742,031, filed Dec. 2, 2005, the entirety of which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the field of nanotechnology and nano-scale materials, and in particular to nano-scale silver platelets and methods for their production.

BACKGROUND OF THE INVENTION

Silver platelets and silver flakes, having superior thermal and electrical conductivities, represent a very important class of materials, which are used in various industries, such as the electronics industry, to generate electrically conductive structures and to provide EMI shielding.

A variety of methods have been used to prepare silver flakes or silver platelets, such as vertical freezing, ball milling, epitaxial growth, gas evaporation, vacuum deposition, Langmuir-Blodgett films, and chemical precipitation. The silver flakes used in the electronic industry are almost exclusively produced by milling silver powders in various solvents in the presence of suitable lubricants, as for example in U.S. Pat. No. 4,859,241. The flattening of the silver particles is the result of mechanical forces (shear and impact) provided by the movement of the milling media, which usually contains 1-5 mm spheres of materials of different densities and compositions (glass, stainless steel, or ceramics). Because the majority of the silver powders used in the milling process contain large agglomerates of submicrometer or micrometer size particles, the milling route almost always leads to the formation of silver flakes with large average particle sizes (5-20 p.m) and broad size distributions. Such materials are becoming less and less suitable for each succeeding generation of electronic devices, which require increasingly thinner and smaller conductive structures. Furthermore, friction- and impact-induced wear of the milling media leads to contamination of the resulting silver flakes, thereby reducing the quality of the product.

Precipitation-based silver flake production may be a promising technology in meeting the demands of the electronic industry. For example, Yener et al. obtained nano-size silver flakes by reducing silver nitrate with hydrazine at the interface of a water/octylamine bi-layer system (Yener et al., Langmuir (2002) 18:8692-99). However, such a system is not environmentally friendly, and use of such a system on a commercial scale would be complex and costly. Silver nano-platelets in the 30-120 nm size range have been produced byirradiation of silver nanospheres (R. Jin el al., Nature (2003) 425:487-490), but this is a twostep process; furthermore, photochemical processes are rarely amenable to commercial-scale production.

Consequently, there is a great interest in the development of new, cost-effective, and environmentally friendly protocols that are capable of producing smaller and more uniform silver flakes.

SUMMARY OF THE INVENTION

The present invention provides a method for producing nanometer-scale silver platelets, which includes the essentially simultaneous addition of a silver ion solution and a reducing solution to an acidic solution under conditions that permit the reduction of silver ions to metallic silver particles, wherein the silver ion solution contains a plurality of silver ions; the reducing solution contains one or more reducing agents; the acidic solution contains one or more stabilizing agents; and at least a part of the reduction of the silver ions and concurrent formation of the silver particles takes place in the presence of a plurality of palladium ions.

In one embodiment, the reducing agent is ascorbic acid or isoascorbic acid. In another embodiment, the stabilizing agent is gum arabic. In yet another embodiment, the silver ion solution contains a plurality of palladium ions, and in still another embodiment, the reducing solution further contains gum arabic as a stabilizing agent.

Also provided are silver platelets produced according to the method of the present invention, and compositions including such silver platelets.

The present invention further provides a metallic platelet containing more than about 99.1% silver and less than about 0.8% palladium, and compositions including the same.

Other features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the FE-SEM images of silver platelets produced in accordance with one embodiment of the present invention. The images were obtained by using a JEOL JSM-7400F system at 15 kV accelerating voltage. Magnifications: (a) 10,000×; (b) 25,000×; (c) 100,000×; and (d) 100,000×.

FIG. 2 illustrates the particle size distribution (PSD) of silver platelets produced in accordance with one embodiment of the present invention. The PSD was determined by laser diffraction using a Malvern 2000 size analyzer.

DETAILED DESCRIPTION OF THE INVENTION

As used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural references, unless the content clearly dictates otherwise. Thus, for example, reference to “a silver platelet” includes a plurality of such platelets and equivalents thereof known to those skilled in the art, and reference to “a reducing agent” is a reference to one or more reducing agents and equivalents thereof known to those skilled in the art, and so forth. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.

The present invention provides a simple, cost-effective, and Environmentally friendly chemical method for producing smaller, thinner, and more uniform silver particles than have previously been available. More specifically, the method of the invention produces flat metallic particles having a thickness of about 10 to about 200 nm, a width of about 100 to about 2000 nm, and an aspect ratio (width/thickness) ranging from about 5 to about 30. Such particles are referred to herein as “nano-platelets”. In some embodiments, a majority of the nano-platelets are characterized by an axis of symmetry, which is typically (but not limited to) 3-fold symmetry.

The inventors have previously demonstrated the preparation of uniform, isometric nano-sized silver particles in acidic solutions, by the reduction of silver ions with ascorbic acid in the presence of various dispersants. Upon further study, the inventors surprisingly found that nano-sized, uniform, anisotropic silver particles, i.e. silver platelets, can be obtained by manipulating the conditions for nucleation and early stages of particle growth in an acidic reducing system.

In at least one embodiment, the present invention provides a method or system which beneficially produces silver platelets, and also the silver platelets produced thereby, that includes essentially simultaneously adding a silver ion solution and a reducing solution to an acidic solution under conditions that permit the reduction of a silver ion to metallic silver, thereby producing the silver platelets.

Unlike the binary system known in the art, where silver platelets are formed at the lamellar bilayer interface of a hydrophobic organic compound-water system, the system of the present invention is a single phase system in which silver platelets are formed in and precipitated out of an aqueous solution.

The silver ion solution contains a plurality of silver ions, and may optionally contain palladium ions. For example, a silver ion solution may contain about 0.94 M Ag⁴ and about 0.02 M Pd^(2÷). In another example, a silver ion solution may contain about 0.99 M Ag÷. The silver ion solution may be prepared using any silver compound(s) known in the art which releases silver ions into solution to be reduced by a reducing agent to form metallic silver particles. In one embodiment, the silver compound may be silver nitrate.

The silver ion solution may be a homogenous solution, a gradient solution, or a combination of solutions continuously or substantially continuously applied during the process of the present invention. A gradient solution contains at least one non-uniformly distributed substance. The gradient may be of any type, for example a linear gradient, a nonlinear gradient, or a step gradient. The non-uniformly-distributed substance may be a plurality of palladium ions. The inventors discovered that if, during the nucleation and the early stages of particle growth, a large excess of nitric acid (0.5-3.0 mol.dm⁻³) is provided and Pd²⁺ ions are present, (introduced, for example, in the silver nitrate solution at 1.0-10% based on the weight of silver), the growth mechanism of the metallic particles changes from isotropic growth to anisotropic growth, and highly dispersed silver nano-platelets are eventually formed. In one embodiment, the silver ion solution may comprise a preceding portion and a succeeding portion, wherein the preceding portion of the silver ion solution contains a plurality of silver and palladium ions. The preceding portion and the succeeding portion may be part of one solution, or be different solutions separately prepared and applied during the process of the present invention.

The reducing solution of the present invention may contain a reducing agent. The term “reducing composition,” or “reducing agent,” as used herein, generally includes any water soluble reducing substance, and combinations thereof, which may be capable of reducing silver ions to metallic silver. Suitable reducing agents include, without limitation, acids, aldehydes, aldoses, monohydroxylic alcohols, polyhydroxylic alcohols (polyols), hydrazine, and reducing saccharides (including monosaccharides, oligosaccharides, and polysaccharides). In one embodiment, the reducing agent may be ascorbic acid or isoascorbic acid. For example, a reducing solution of the present invention may be prepared by dissolving 32 g ascorbic acid in 160 ml H20.

The reaction mixture of the present invention may contain a stabilizing agent. The stabilizing agent may be present in one or more of the silver ion solution, the reducing solution, and the acidic solution. In one embodiment, both the reducing solution and the acidic solution contain a stabilizing agent.

The term “stabilizing composition,” or “stabilizing agent,” as used herein, generally includes any water soluble stabilizing substance which is capable of dispersing and stabilizing the newly formed silver nano-platelets in the reaction mixture, and thus preventing undesirable aggregation of these particles. Suitable stabilizing agents are known in the art, and include, without limitation, water soluble resins (including, e.g., naturally occurring, synthetic, and semi-synthetic water soluble resins), gum arabic, water soluble polymers, water soluble polysaccharides, water soluble glycoproteins, various water soluble salts of naphthalene sulfonic-formaldehyde co-polymers, and combinations thereof. In one embodiment of the present invention, the stabilizing agent is gum arabic.

The stabilizing agent may be removed after the formation of the nano-platelets is complete. A number of protocols for removing the stabilizing agent are known in the art, such as, acid, alkaline, and/or enzymatic hydrolysis. In one embodiment, gum arabic may be removed from the reaction mixture after the reaction through alkaline hydrolysis. For example, the hydrolysis may be performed for an extended time at high temperature (e.g. between about 70° and about 100° C., or between about 80° and about 90° C., or between about 82° and about 88° C.) and at high pH (e.g. pH 11.5). It is generally desirable to maintain the pH of the mixture during the hydrolysis at between about 9 and about 14, or between about 10 and about 12, or between about 10.5 and about 11.5. The duration of the hydrolysis may depend upon a number of factors, such as temperature, pH, and the amount of stabilizing agent (e.g. gum arabic) present. For example, the hydrolysis of gum arabic may generally be performed for about 0.2 to 10 hours, or about 1 to 5 hours, or about 2 to 3 hours.

The acidic solution of the present invention may contain an acid and optionally, a stabilizing agent. For example, the acidic solution may be prepared by dissolving 2.3 g of gum arabic in 320 ml H₂O, followed by the addition of 20 ml of concentrated HNO₃. In certain embodiments, the concentration of the nitric acid may be from about 0.5 to about 3.0 M.

The resulting silver platelets may be isolated following standard protocols known in the art, such as by precipitation, filtration, and centrifugation. In one embodiment, the utilization of centrifugation may facilitate the production of silver platelets with a smaller size. For example, the reaction may be carried out in a continuous centrifuge under conditions wherein the silver platelets with a smaller size may be precipitated without causing substantial aggregation of the platelets. The particles may then be washed, for example by using methanol or ethanol, and dried, such as by air or N2, or under vacuum.

In one embodiment, the silver platelets may contain about 99.1% or more silver, and about 0.8% or less palladium. In another embodiment, the silver platelets may contain about 0.2% palladium. The incorporation of a trace amount of palladium into the silver platelet of the present invention may significantly reduce the migration problems which are known to affect all silver-based particles, flakes, and platelets.

The silver platelets produced in accordance with the present invention may have a thickness of about 10 nm to about 200 nm, or about 40 to about 100 nm. In one embodiment, the silver platelets may have an average thickness of about 60 nm. The silver platelets of the present invention may be highly uniform in size. For example, at least about 50%, or in some cases at least about 90%, of the silver platelets may have a thickness within a range of T±0.2 T, wherein T is the average thickness of the silver platelets. In some embodiments, at least about 50%, or in some cases at least about 90% the platelets, have a thickness within the range T±0.1 T.

The silver platelets produced in accordance with the present invention may have a width of about 100 nm to about 1200 nm. In one embodiment, the silver platelets may have an average width of about 500-1000 nm, or 700-800 nm. In some embodiments, at least about 50% or in some cases at least about 90%, of the silver platelets may have a width within a range of W*0.2 W, wherein W is the average width of the plurality of silver platelets. In other embodiments, at least about 50%, or in some cases at least about 90% the platelets, have a width within the range W±0.1 W.

The uniformity of the silver platelets produced in accordance the present invention may also be characterized by the aspect ratio (width/thickness) of the platelets. The aspect ratio of the silver platelets may range from about 3 to about 30. For example, at least about 50%, or in some cases at least about 90%, of the silver platelets may have an aspect ratio of about 12. In addition, the silver platelets of the present invention may be highly crystalline.

The silver nano-platelets of the present invention may be substituted, wholly or in part, for prior art silver flake and silver powder compositions in many applications. For example, conductive inks, coatings, and adhesives may comprise silver nano-platelets according to the invention, together with thermoplastic and/or thermosetting polymers, solvents, and various additives such as binders, stabilizers, rheological modifiers, and surfactants. Solvents, polymers, and additives suitable for use in conductive inks, coatings, and adhesives are well-known in the art; see for example U.S. Pat. No. 6,379,745 (which is incorporated herein by reference) and references cited therein.

EXAMPLES

The following examples illustrate the present invention, which are set forth to aid in the understanding of the invention, and should not be construed to limit in any way the scope of the invention as defined in the claims.

Materials: Silver nitrate (AgNO₃) was obtained from Ames Goldsmith Corp. (Glens Falls, N.Y.). Palladium nitrate (Pd(NO₃)₂) was obtained from Umicore (Belgium). Gum arabic was obtained from Frutarom Incorporated (North Bergen, N.J.). Nitric acid (HNO₃) was obtained from Fisher Scientific Co. (Fair Lawn, N.J.). Ascorbic Acid (C6F1s06) was obtained from Roche Vitamins Inc. (Parsippany, N.Y.). Deionized (“DI”) water was used throughout.

Example 1—Preparation Of Silver Platelets (A) Precipitation Process

The silver nano-platelets were prepared by the double-jet addition of equal volumes of silver nitrate (AgNO₃) and ascorbic acid solutions into a highly acidic solution of gum arabic in water. To generate the metallic particles as very small nano-platelets, the total volume of silver nitrate solution (160 ml) was divided into two distinct parts (A₁ and A2). The first part of the solution (AO was prepared by dissolving 1.91 g AgNO₃ in 11.14 ml H2O and then adding 0.275 g Pd(NO₃)₂ solution (9.0% Pd metal) to provide a concentration of 0.94 M Ag and 0.02 M Pd (final volume: 12 mi). The second part (A₂) was prepared by dissolving 23.24 g AgNO₃ in H₂O (final volume: 148 ml), to provide an AgNO₃ solution with a concentration of 0.99 M Ag. The reducing solution was prepared by dissolving 32 g ascorbic acid and 1.5 g gum arabic in 160 ml H₂O. The acidic solution was prepared by dissolving 2.3 g gum arabic in 320 ml H₂O, followed by the addition of 20 ml of concentrated HNO3.

The precipitation was carried out by the addition, in parallel, of the metallic precursor solutions (first the A_(i) solution followed immediately by the A2 solution) and the ascorbic acid solution, over 90 minutes, into the 320 ml of acidified gum arabic solution, at room temperature and with moderate agitation. When the addition of both solutions was completed, the temperature of the reaction mixture was brought to 80° C. The process was conducted under continuous stiffing.

It is preferable that the time taken for introducing the reducing solution containing ascorbic acid, and the total time taken for introducing the solutions containing silver ions, do not differ by more than 3 minutes.

(B) Hydrolysis of Gum Arabic

The excess of gum arabic was removed by increasing the pH of the silver platelet dispersion to 11.5 with 10.0 N sodium hydroxide. The dispersion was maintained at a temperature of about 85° C. for 2.5 hours.

(C) Processing the Silver Platelets

When the hydrolysis of the gum arabic was complete, the dispersion was allowed to cool and the silver platelets allowed to settle. The supernatant was discarded and the silver platelets were washed two times with DI water. A third wash was carried out with 50% ethanol in DI water, and two more washes with pure alcohol were performed. The platelets were then allowed to dry overnight on filter paper at room temperature.

(D) Platelet Characterization

The morphology and size of the silver nano-platelets produced in accordance with the methods of the present invention were analyzed by field emission scanning electron microscopy (FE-SEM) using a JEOL JSM-7400F device at 15 kV accelerating voltage and a magnification between 10,000× and 650,000×.

FIG. 1 shows the FE-SEM images of the silver platelets obtained by the process described above. A large majority of the metallic particles were crystalline silver platelets, with an average thickness of about 60 nm and an average width of 0.725 gm, for an average aspect ratio (width/thickness) of 12.

The particle size distribution was further confirmed by the laser diffraction technique using a Malvern 2000 size analyzer (FIG. 2), which indicated an average particle size of about 0.8 gm.

While the foregoing invention has been described in some detail for purposes of clarity and understanding, it will be appreciated by one skilled in the art, from a reading of the disclosure, that various changes in form and detail can be made without departing from the true scope of the invention as set forth in the appended claims. 

What is claimed is:
 1. A method for making silver nano-platelets with an aspect ratio of at least 2, comprising the essentially simultaneous addition of a silver ion solution and a reducing solution to an acidic solution, under conditions that permit the reduction of the silver ions to metallic silver, wherein (a) the silver ion solution comprises a plurality of silver ions; (b) the reducing solution comprises one or more reducing agents; (c) the acidic solution comprises one or more stabilizing agents; and (d) at least part of the reduction takes place in the presence of palladium ions.
 2. The method of claim 1, wherein the reducing solution comprises ascorbic acid, isoascorbic acid, or a salt thereof.
 3. The method of claim 1, wherein at least a portion of the silver ion solution further comprises palladium ions.
 4. The method of claim 1, wherein the silver ion solution is divided into a preceding portion and a succeeding portion, and wherein the preceding portion of the silver ion solution comprises palladium ions.
 5. The method of claim 1, wherein the total amount of palladium, relative to the total amount of silver, is about 0.2% by weight.
 6. The method of claim 1, wherein the total amount of palladium, relative to the total amount of silver, is at least 0.05% by weight.
 7. The method of claim 1, wherein the acidic solution comprises gum arabic.
 8. A silver nano-platelet obtained in accordance with the method of claim
 1. 9. A silver nano-platelet obtained in accordance with the method of claim
 7. 10. A plurality of the silver nano-platelets obtained in accordance with the method of claim 1, wherein at least 90% of the silver platelets have a width within the range of W±20% W, where W is the mean width of the silver platelets.
 11. The plurality of silver nano-platelets of claim 10, wherein at least 80% of the silver platelets have a thickness within the range of T±20% T, where T is the mean thickness of the silver platelets. 